It is desired that a situation on a road where vehicles are traveling be measured for each lane to detect the flow or speed of vehicles for each lane, such that a signal control or a patrol of vehicles will be improved or rear-end collision accidents will be prevented by providing successive vehicles with information.
As one technique that responds to such demands, a technique is known in which the traffic lane of a vehicle is identified by identifying the position of a vehicle on a polar coordinate system with the center being the position at which a radar device is installed, where radar devices capable of sweeping the irradiation directions of radar waves are used to sweep the radar waves along the road width. However, such sweeping radar devices have a disadvantage in durability and in the maintenance of sweeping angles in consideration of the fact that the devices are continuously used for a long period of time after the installation. For this reason, it is desirable that a situation in which vehicles are traveling be detected for each lane by using non-sweeping radar devices which are installed for each lane, i.e., radar devices capable of detecting the distance between the radar device and a vehicle as well as the traveling speed of a vehicle but not capable of detecting the existing direction of a vehicle.
As a first technique that responds to such demands, a technique is known in which the lane where the vehicle detected by a radar device is traveling is distinguished. This technique is provided for each of two or more lanes to detect a vehicle by using the information from two or more radar devices whose detection range includes a lane adjacent to the installed lane. In this technique, a detected distance indicating the distance between a detected object and the radar that detected the object as well as the received power from the object are acquired from each radar device. Next, cases are detected where the distance between a first object detected by a first radar out of the two or more radar devices and a second object detected by a second radar installed for the lane adjacent to the lane in which the first radar is installed is equal to or less than a threshold. Then, the radar device that acquired the larger received power therein from the first or second object is identified, and it is determined that the vehicle is traveling in the lane in which the identified radar device is installed.
Moreover, as a second technique that responds to the demands, a technique is known in which the occurrence of traffic congestion, traffic accidents, or the like on the road, or the behavior of an abnormal vehicle expected to cause traffic accidents, is detected. In this technique, two or more radar devices are used that are installed in each lane and that detect the intensity or spectrum of a signal reflected from a vehicle on a lane. In this technique, the intensity or spectrum of a signal reflected from a vehicle detected by the radar devices is used to calculate the position and speed of the subject vehicle in the lane direction. Here, when at least two radio radars out of the radio radars installed for each lane detect a reflected signal from the same traveling vehicle, such a vehicle is identified as the same vehicle. Next, the maximum value in the amplitude of a reflected signal from the same traveling vehicle identified as the same vehicle is obtained for each certain period of time determined in advance for each radar device, and the obtained maximum values are compared with each other, thereby estimating the lane in which the vehicle exists or the position in the road width direction. Accordingly, a two-dimensional position or behavior of a vehicle on the road is estimated on the basis of the above-obtained information of the position and speed of the two or more different vehicles in the lane direction as well as the position in the road width direction.
The coverage of radar devices used in the first and second techniques (the area of a position in which a vehicle exists where the radar wave emitted from the radar device is reflected and it is expected that the reflected wave from a vehicle at a received power greater than a threshold will be received by the radar device) has an elliptic shape rather than a rectangular shape. For this reason, if the detection range where it is expected that a vehicle will be detected by using a radar device is expanded to the full width of a lane in order to detect a vehicle traveling at an edge of the lane, even a portion of the lane next to the lane to be detected is included in the detection range.
FIG. 1 is a schematic diagram illustrating the relationship between the detection range of radar devices and lanes, where two radar devices 1a and 1b are installed for two adjacent lanes 2a and 2b, respectively. If detection ranges 3a and 3b of the respective radar devices 1a and 1b are expanded in the width direction of the lanes 2a and 2b, the radar device 1a would detect, near an overlapping detection range 4, even a vehicle that is traveling in the lane 2b next to the lane 2a in which the radar device 1a is installed. Moreover, the radar device 1b also detects a vehicle that is traveling in the lane 2a next to the lane 2b in which the radar device 1b is installed.
In view of such problems, in the aforementioned first and second techniques, the output values of the two radar devices that detect the respective adjacent lanes are compared with each other to determine in which of the adjacent lanes the vehicle is traveling.
Moreover, in the aforementioned first and second techniques, how the received power of the reflected wave of a radar wave from the same vehicle changes overtime is monitored in order to determine the traffic lane of a vehicle. In other words, in these techniques, the received power of a reflected wave from the same vehicle is tracked on a time-series basis.
As another background art, a technique is known that is not dependent on weather conditions or the like, and that measures the absolute positions of vehicles on wide areas of the road on a real time basis. In this technique, a radar transmitter that transmits a radar wave to a specified section on the road, and a radar receiver that receives the reflected wave of a radar wave transmitted from the radar transmitter are used. In this technique, the positions of vehicles on a coordinate system with reference to the ground including the surface of the road in the specified section are firstly obtained from the positions on the specified section of the road according to the reflected wave received at the radar receiver. Then, the obtained position information is transmitted by radio to vehicles that are traveling in the specified section, and is displayed on display devices on the road or on surveillance display devices.
Note that the techniques disclosed in the following documents are known.                Document 1: Japanese Laid-open Patent Publication No. 2011-196885        Document 2: Japanese Laid-open Patent Publication No. 2000-048296        Document 3: Japanese Laid-open Patent Publication No. 11-086183        
The techniques of detecting a vehicle by using a radar device may be classified into backward detection and forward detection according to the difference in the position of a radar device with reference to the direction in which a vehicle is traveling. Backward detection and forward detection will be explained with reference to FIG. 2.
Backward detection is a technique in which a vehicle 5 in a detection range 3 traveling in a direction moving away from a radar device 1 (the direction indicated by an arrow) is detected by the radar device 1 from the rear of the vehicle 5. Moreover, forward detection is a technique in which the vehicle 5 in the detection range 3 traveling in a direction approaching the radar device 1 (the direction indicated by an arrow) is detected by the radar device 1 from the front of the vehicle 5.
In backward detection, the position at which the radar device 1 catches the vehicle 5 for the first time is a position closest to the radar device 1 in the detection range 3 of the radar device 1. Accordingly, a strong reflected wave is obtained from the vehicle at such a position, and a result of measuring the position of the vehicle 5 (distance from the radar device 1) and the speed of the vehicle 5 by the radar device 1 is expected to be highly accurate, and the reliability is high. Subsequently, as the distance from the radar device 1 becomes larger due to the movement of the vehicle 5, the resolution power of the measurement of distance or speed degrades. As the vehicle 5 moves away from the radar device 1, the intensity of the reflected wave also become weak, and the influence or the like of several types of noise or the reflected wave coming through a different path become relatively large. As a result, the accuracy of the measurement result degrades.
When the tracking of the received power of a reflected wave from the same vehicle 5 on a time-series basis is performed by using backward detection, the reflected wave at the time when the radar device 1 caught the vehicle for the first time is strong. Accordingly, it is possible to detect the traffic lane of the vehicle 5 according to the intensity of the reflected wave at a satisfactory level. As the reliability of the measurement result of the position and speed of the vehicle 5 at this stage is also high, the subsequent tracking of the vehicle 5 is relatively easy.
On the other hand, if the tracking is performed by using forward detection, the position at which the radar device 1 catches the vehicle 5 for the first time is a position furthest from the radar device 1 in the detection range 3 of the radar device 1. As the reflected wave from the vehicle 5 at this position is weak, the detection of the traffic lane of the vehicle 5 according to the intensity of the reflected wave may become difficult. Moreover, as the reliability of the measurement result of the position and speed of the vehicle 5 at this stage is also low, the subsequent tracking of the vehicle 5 will be difficult for a certain period of time.
As described above, when the vehicle 5 is detected by using the radar device 1, the backward detection is generally more advantageous than the forward detection. However, there are many cases where the vehicle 5 needs to be detected by using the forward detection due to some constraints on the position at which the radar device 1 is to be installed.